The present invention relates to a semiconductor memory device and, more particularly, to a semiconductor memory device which includes a control circuit for reading out stored data from a fuse circuit using an electrically erasable/programmable nonvolatile semiconductor memory cell (EEPROM cell) as memory element for function control data and is used for a batch erasable semiconductor memory such as a flash EEPROM.
EEPROMs have advantages, e.g., the advantage that data in memory cells are not erased after the power is turned off, and hence the demand for these memories have recently increased greatly. A batch erasable flash type EEPROM (flash memory), in particular, in which each memory cell has one transistor, is expected to replace a large-capacity magnetic disk.
In a conventional flash memory, each memory cell used in a cell array includes an NMOS field effect transistor having a stacked gate structure in which a floating gate formed as a charge storage layer and a control gate are stacked in a gate insulating film.
In such a flash memory, a ROM cell that is formed by the same process as that for a cell transistor and has the same structure as that thereof is sometimes used as a memory element (fuse element) storing, for example, redundancy control data, trimming control data, and write-protect control data which are used to control a redundancy function, a trimming function, and a write-protect function.
The redundancy function control data is data for indicating substitute addresses (fail addresses in a main memory cell array) in a redundancy circuit which is used as a measure against fail bits in a main memory cell array in a current large-capacity flash memory.
The trimming data is data for adjusting a trimming circuit used to generate a reference voltage serving as a reference for a write voltage, an erase voltage, and like in a flash memory using a single power supply.
The write-protect control data is data for controlling to inhibit rewriting of data in a designated block in a flash memory using an arrangement in which a memory cell array is divided into blocks.
As described above, the electrical characteristics (the charge amount of the floating gate) of each ROM cell used as a fuse element used to store function control data in the conventional semiconductor memory device vary with time when a voltage for reading out stored data is kept applied to the drain of the fuse element ROM cell. As a result, the stored data may change from the initial state at the start of the use of the device.